New Research Shines Light on Black Hole Accretion Discs

By: CofC Media

Back holes, the collapsed remnants of stars at least 8 times more massive than the Sun, are some of the most exotic and powerful objects in the Universe. Matter falling into them, in a process called accretion, heats up to tremendous temperatures under the strong gravitational influence of the black hole. Some of the energy released in this process is radiated away, primarily in the form of X-rays, before the matter completes its plunge through the event horizon. The X-rays emitted from the infalling matter carry information about the black hole and its surrounding environment, making their collection and measurement a primary mission of NASA and other space agencies for more than 4 decades.

The X-ray radiation coming from most accreting black holes can roughly be divided into 2 components, one called the “soft” component (associated with lower energies) and another called the “hard” component (associated with higher energies). One notion for why there are 2 X-ray components is that the accretion flow itself might be divided into 2 distinct segments, a relatively cool, thin disk, responsible for the soft X-rays, and a much hotter, thicker corona, responsible for the hard X-rays.

In the 1980s, astronomers discovered that the X-rays, particularly the ones coming from the corona, flicker at certain frequencies, a phenomenon known as quasi-periodic oscillation (QPO). One proposed explanation for the flickering is that the corona wobbles around, like a spinning top, due to something called Lense-Thirring precession. This idea was first proposed about 15 years ago. One problem with this picture, though, was that the frequency of the precession seemed to be too high to match the observed QPOs. However, those earlier predictions only considered the precession of an isolated corona, without an accompanying thin disk. Recent, state-of-the-art computer simulations that include both the corona and the thin disk have demonstrated that the presence of the disk significantly slows down the precession, relieving much of the tension between this model and observations. “This is very exciting, as precession can now reproduce the QPO frequencies we actually see,” says Oxford astronomer, Dr. Adam Ingram, who was not affiliated with this work.

“The study of X-ray variability around accreting black holes has long held out the promise to help us better understand that environment; these results hopefully get us one step closer to realizing that promise, “ says College of Charleston astronomer, Prof. Chris Fragile.

These results may also have important implications for studies of black hole properties since the observed precession frequency can be used along with this model to infer such things as the black hole’s mass and spin. “This gives us another tool to try to pin down the spin of black holes, and so, serves as a nice consistency check for other methods,” says University of Maryland astronomer, Prof. Chris Reynolds.

One interesting feature of this work is that the precession only happens for accretion flows that are tilted, or misaligned, with the spin axis of the black hole. The magnitude of this tilt may tell astronomers something about how black hole systems form and evolve.

This work was carried out by Dr. Deepika Bollimpalli at the Max-Planck-Institut fur Astrophysik in Garching, Germany; Prof. Chris Fragile at the College of Charleston; and Prof. Wlodek Kluzniak of the Nicolaus Copernicus Astronomical Center in Warsaw, Poland. The simulations used resources at the College of Charleston, the Texas Advanced Computing Center, Prometheus supercomputing cluster of PL-Grid infrastructure in Poland, and MPCDF clusters in Germany.

The results have been published in the Monthly Notices of the Royal Astronomical Society.

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